1. Field of the Invention
This invention relates to the manufacture of silica optical fiber preforms.
2. Description of the Prior Art
In the method of optical fiber preform manufacture to which this invention relates, the silica tube, complete with the coating lining its bore, is heated to cause the tube to soften and its bore collapse by the effect of the forces of surface tension. For this purpose a source of heat, such as an oxyhydrogen flame, provides a localized hot zone which is slowly traversed along the tube to effect localized softening of the tube wall. Collapse is performed by a number of passes of the hot zone, whereby the bore of the tube is progressively reduced in diameter until on a final pass the bore is completely collapsed, thereby creating a solid optical fiber preform from which optical fiber may subsequently be formed by a drawing down operation.
With this method of manufacture there is the general problem that as the tube softens, and its bore begins to collapse, the surface tension forces tend to seek the minimum energy configuration. This unfortunately is not one of circular symmetry, but rather is one in which the tube bore is flattened out.
A second problem occurs if the core material contains a volatile dopant such as germania and/or phosphorus pentoxide, and concerns simply the fact that the high temperatures necessary to soften the tube wall may also be sufficient to cause appreciable volatilization and loss of the volatile dopant from the bore surface, thereby modifying the refractive index profile of the resulting preform.
To obtain a small pulse dispersion in a fiber it is necessary to grade the refractive index profile across the core region to a near-parabolic shape. The concentration of refractive index enhancing dpoant should be low at the core periphery, and should increase in a graded manner to reach a maximum at the center. Anything which causes a deviation from the ideal profile is detrimental to bandwidth capability. Taking by way of example the case where germania is used as a refractive index enhancing dopant, it is found that at the temperatures necessary to soften the silica tube for collapse (2000.degree.-2200.degree. C.) germania starts to be lost by volatilization via the sub-oxide according to the reaction: EQU GeO.sub.2 .revreaction. GeO+ 1/2 O.sup.2
if the deposited layer lining the bore of the tube is provided with a refractive index grading that, in the absence of volatilization effects, would produce the optimum grading in the preform, then the effect of volatilization is to produce a lowering of the refractive index at the center of the preform core as depicted in FIG. 1a. A further effect of volatilization is to produce a longitudinal variation in refractive index profile. Germania is lost from the tube wall at the hot zone via the sub-oxide, but, since this is a reversible reaction, some of this oxide recondenses on cooler regions of the bore ahead of the moving hot zone. Some of this condensate is re-incorporated by diffusion into the glass as the hot zone advances, and this can give rise to the formation of a central peak within the trough in the refractive index at the center of the core of the preform. This peak is highest at the end longest exposed to the recondensation of the dopant, and can, at this end, produce a refractive index profile as depicted in FIG. 1b.